updog: Set permissions and extension of migrations

[sam-aws]

Issue #, if available:
Fixes #558

Description of changes:
When downloading migrations don't include the '.lz4' extension in the resulting file, and make sure the file is executable.

Signed-off-by: Samuel Mendoza-Jonas [email protected]

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

Tested by checking the migration binaries after an update, before rebooting:

bash-5.0# updog check-update
aws-k8s-0.2.0 (v0.1)
bash-5.0# updog update-image
Starting update to 0.2.0
Update applied: aws-k8s-0.2.0
bash-5.0# ls -l /var/lib/thar/datastore/migrations/
total 1048
-rwxr-xr-x 1 root root 555720 Nov 25 21:30 borkseed-migration
-rwxr-xr-x 1 root root 514760 Nov 25 21:30 host-containers-version-migration
bash-5.0# /var/lib/thar/datastore/migrations/borkseed-migration
Usage: /var/lib/thar/datastore/migrations/borkseed-migration
            --datastore-path PATH
            ( --forward | --backward )